Normally, the force acting to open the tube of an expansible tube valve comes from the differential between the upstream and downstream pressures. Should this differential become low, as when the demand for flow is high, it may be insufficient to open the tube fully against the elastic forces of the tube. Hence, at the time when full expansion of the tube is most needed, the forces available to produce the expansion are at a minimum.
In Gerbic U.S. Pat. No. 3, 669,142 this problem is dealt with in two ways. First, it proposes discharging the jacket to a region immediately downstream of the valve barrier, where static pressure is lower than elsewhere downstream, thus providing an added margin of differential to facilitate expansion of the tube. However, even this added differential may not always be sufficient to open the tube fully. Second, Gerbic proposes discharging the pilot to the atmosphere, but, where natural gas is the medium, this can only be done where atmospheric leak is safe and acceptable from an environmental standpoint.